The invention relates to a coupling for a belt retractor incorporating a belt tensioner.
Such a coupling usually comprises a hub adapted to be driven by the belt tensioner and has several contact surfaces, and comprises several clamping rollers that are arranged in the hub in such a way that they are adapted to be urged radially inwards by the contact surfaces, and a shaft adapted to be coupled to the hub by means of the clamping rollers. The belt tensioner serves to tension the vehicle occupant seat belt that is accommodated on a belt spool of the belt retractor when a vehicle accident is detected by a suitable sensor. By tensioning the belt, the so-called belt slack is eliminated so that the buckled-up vehicle occupant participates in the deceleration of the vehicle as soon as possible. The coupling has to fulfill various functions. For one thing, in the starting position, it has to separate the belt tensioner from the shaft that is connected to the belt spool so that the belt spool can rotate freely and so that belt webbing can be unwound from the spool and then wound up on it again. Secondly, when the belt tensioner is activated, it has to establish a connection between the belt tensioner and the shaft that is connected to the belt spool of the belt retractor with as little slippage as possible, so that the tensioning distance provided by the belt tensioner is transferred to the belt spool without losses. The term tensioning distance is used to refer to the length of the belt webbing that is wound up on the belt spool when the belt tensioner is triggered. Finally, the coupling has to once again release the connection between the belt spool and the belt tensioner so that the belt spool can rotate again and the belt retractor can be used once again.
With the couplings known so far, normally three rollers or balls are used which, when the belt tensioner is activated, are clamped between the contact surfaces of the hub and the shaft. Although such a clamp roller coupling reliably couples the hub and the shaft when the belt tensioner drive is activated, it is not possible, however, to release the coupling after the tensioning procedure has ended since the clamping bodies are clamped between the contact surfaces and the shaft in a self-inhibiting manner. This prevents the belt spool of the belt retractor from being able to rotate freely again after a tensioning of the belt webbing.
The object of the invention is to improve a coupling of the above-mentioned type in such a way that the coupling can be reliably released again after the tensioning procedure has ended.
This is achieved in a coupling which comprises a hub adapted to be driven by the belt tensioner and has several contact surfaces, and comprises several clamping rollers that are arranged in the hub in such a way that they are adapted to be urged radially inwards by the contact surfaces. The coupling further comprises a shaft which is adapted to be coupled to the hub by means of the clamping rollers. At least one return element is provided that urges the clamping rollers into a position out of engagement with the shaft. On the one hand, the return element is configured in such a way that, at the beginning of the tensioning procedure, it makes it possible to bring the clamping rollers into such a position between the contact surfaces and the shaft that the hub is reliably coupled to the shaft. On the other hand, the return element is configured in such a way that, when the coupling is released after a tensioning procedure, for example, with the tension in the tensioned belt webbing lessening, the clamping rollers are again moved into their starting position in which they do not engage the shaft. As a result, the belt spool is separated from the belt tensioner again, so that the former can rotate freely.
Preferably, it is provided for that the return element is a spring cage that urges the clamping rollers into the position out of engagement from the shaft. The use of a spring cage makes it possible to mount the clamping rollers in the hub with very little effort, since the clamping cage, together with the pre-assembled clamping rollers, can be installed as one single component. This results in low production costs.
In order to improve the performance of the coupling, it is provided for that the hub is configured with external teeth on which the belt tensioner engages and that the external teeth, as seen in the axial direction, lie within the area defined by the clamping rollers. In other words, the clamping rollers, as seen in the radial direction, are arranged below the teeth and are not staggered in the lengthwise direction with respect to the teeth. This arrangement ensures the shortest possible force flow path from the teeth of the hub to the shaft and prevents the hub from tilting when it engages the clamping rollers, which would have a detrimental effect on the force transmission. Moreover, a tilted hub entails the risk that the clamping rollers will not be released properly after the tensioning has taken place.
Advantageous embodiments of the invention will be apparent from the subclaims.